This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-334467 filed on Nov. 25, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a rotation detector that detects an angular position or a rotational speed of a rotating object, the rotation detector using magnetoresistive elements that change their electrical resistance in response to changes in a magnetic field vector. The rotation detector is suitable for use in an engine control and an anti-block braking system in an automobile vehicle.
2. Description of Related Art
It is generally known to use a bridge circuit having magnetoresistive elements in detecting rotation of a rotating object such as a toothed rotor. The magnetoresistive element (hereinafter referred to as MRE) changes its electrical resistance in response to changes in a biasing magnetic field caused by rotation of a toothed rotor. A pair of MRE circuits are positioned at both sides of a center of a biasing magnetic field. An MRE bridge composed of the pair of MRE circuits detects a rotational position of the toothed rotor.
An integrated circuit chip (herein after referred to as an IC-chip) having an MRE bridge is usually mounted on a copper reed frame and molded with thermosetting resin such as epoxy resin. The molding die is heated to a level of 150-160xc2x0 C. and molten resin at about 175xc2x0 C. is injected into the die to mold the IC chip. A stress is generated inside the IC-chip by the heat in the molding process. That is, an expansion stress is imposed on the IC-chip because the expansion coefficient of the copper reed frame is much higher than that of the IC-chip (made of silicon). Then, the IC-chip molded together with the copper reed frame is cooled down to a room temperature, and a shrinkage stress is given to the IC-chip by shrinkage of the copper frame and molding resin.
Further, the rotation detector is subjected to temperature changes when it is actually used in an automobile, for example. The molding resin expands or shrinks according to the temperature changes. When the temperature rises, for example, the stress imposed on the IC-chip tends to be reduced because the temperature becomes closer to the molding temperature.
Such temperature changes adversely affect the MRE bridge formed in the IC-chip due to magnetostrictive effects caused by a stress imposed on a ferromagnetic thin film such as an MRE. The magnetostrictive effects include changes of magnetic properties such as magnetic field saturation and a resistance-change-ratio in the MRE. The magnetostrictive effects are not uniformly given to both MRE circuits composing the MRE bridge in the conventional rotation detector. Therefore, output voltages of both MRE circuits are differently affected by the magnetostrictive effects, and accordingly, rotation of the toothed rotor cannot be accurately detected.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved rotation detector that accurately detects rotation of a rotating object by eliminating the adverse effects of the magnetostriction.
The rotation detector is composed of a biasing magnet for generating a biasing magnetic field and an MRE bridge formed on an IC-chip. The rotation detector is positioned to face a rotating object such as a toothed rotor with a certain air gap. The MRE bridge includes a first MRE circuit and a second MRE circuit, each circuit having a pair of MREs connected in series between a power source and a ground. The biasing magnet is positioned so that its magnetic field center line coincides with a diameter of the toothed rotor, and the first and second MRE circuits are positioned symmetrically to each other with respect to the magnetic field center line. Electrical resistance of each MRE changes in response to changes in the biasing magnetic field caused by rotation of the toothed rotor. Rotation of the rotor is detected based on output potentials of the MRE bridge.
The resistance of each MRE is influenced by magnetostrictive effects due to external force imposed on the IC-chip on which the MRE is patterned. Such external force is generated by temperature changes or other causes. Since the first and second MRE circuits in the MRE bridge are positioned symmetrically to each other with respect to the biasing magnetic field center line, the influence of the magnetostrictive effects is canceled in the MRE bridge as a whole.
The four MREs in the MRE bridge can be variously patterned, as long as the influence of the magnetostrictive effects is equally given to the first and second MRE circuits, and the influence is canceled in the MRE bridge as a whole. The MREs connected to the power source in both the first and second MRE circuits are positioned symmetrically to each other with respect to the magnetic field center line, and the grounded MREs in both circuits are also symmetrically positioned. A power source side MRE in the first MRE circuit and a grounded MRE in the second MRE circuit may be patterned at one side of the magnetic field center, while a power source side MRE in the second MRE circuit and a grounded MRE in the first MRE circuit may be patterned at the other side of the magnetic field center. Each of the four MREs may be divided into two equal portions, and the power source side MRE and the grounded MRE in both the first and second MRE circuits may be patterned in an X-shape, respectively. It is necessary to select directions of MRE patterning in any arrangement, so that two output potentials of the MRE bridge change with a phase difference therebetween in response to the biasing magnetic field changes.
Since an adverse influence of the magnetostrictive effects on the MRE bridge is canceled as a whole according to the present invention, rotation of the rotating object is precisely detected without being affected by temperature changes or other external disturbances.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.